The present invention relates in general to armable frequency counters for measuring the frequency of an oscillating electrical signal burst and in particular to a frequency counter system which generates its own arming signal derived from the signal burst being measured.
Frequency counters typically determine the frequency of a pulsed signal in two ways. In the first method, the counter counts the number of reference clock events (which may be pulses) of known frequency that occur between successive input signal events. The signal frequency is then computed by dividing the known frequency of the reference signal by the number of counted reference events. In the second method, the counter counts input signal events occurring during a known reference period. The frequency of the input signal is then computed by dividing the number of counted input events by the known reference period. In either case the result may be used to drive a digital display, the display being updated at the end of each count cycle. In prescaled frequency counters, the signal to be measured is first applied to a frequency divider which outputs a pulse to the counter input upon receipt of every Nth signal event where N is a selected integer. Prescaling an input signal to a frequency counter permits higher frequency signal bursts to be measured by slower frequency counters.
Armable frequency counters typically begin operating only after receiving an arming signal from external circuits. As long as a counter is armed it will continue to count input or reference events, computing the signal frequency and updating the display at the end of each count cycle. After the arming signal is turned off, the counter will typically count until the next input signal event occurs and then stop counting until it is rearmed.
To minimize the length of the shortest signal burst which can be measured by an armable frequency counter, the counter should be armed before the signal burst begins. It is also important that the arming signal end before occurrence of the last input signal event. This insures the frequency computed by the counter provides a true reading and is not altered by events occurring after the end of the signal burst. In the prior art, short signal bursts were difficult to frequency measure unless a variable width pulse properly synchronized with the signal burst happened to be available for use as an arming signal. However such a useful variable width pulse was not always present. If the signal burst occurred at regular intervals and was of regular length, an arming signal of proper length and timing could sometimes be generated by adjusting a squarewave signal generator while comparing the signal burst and the arming signal on a dual trace oscilloscope. However, this method is difficult and time consuming. Finally, the method of the prior art is of no use when a frequency counter is to be armed for a single, non-reoccurring signal burst of indeterminate starting and ending time.
Therefore what is needed is a self-arming frequency counter system for measuring the frequency of a prescaled or non-prescaled signal burst wherein counter arming and disarming anticipates the beginning and end of the signal being tested.